This proposal is a response to an RFA from the National Cancer Institute "to solicit (cooperative agreement) applications from investigators qualified to develop and implement a randomized, controlled clinical trial to study the effect of a low-fat diet (20% of dietary calories) on breast cancer incidence," a Phase III trial. Our response is for the development of a Clinical Unit. The initial phase will be that of protocol writing, accomplished jointly by the selected investigators and the NCI staff. This will be followed by a feasibility study and, if appropriate, full implementation. Considerations for protocol development for this study will include: (A) the identification of factors of increased risk for breast cancer and a cohort of women who meet these criteria; (B) clinical trial recruitment mechanisms; (C) clinical criteria for eligibility; (D) time table for study accrual; (E) baseline and follow-up data collection components and mechanisms; (F) mechanisms for monitoring dietary compliance; (G) quality control of clinical trial study components. In general, research methods will include: methods for the identification of approximately 200 women who are believed to be at increased risk for the development of breast cancer (i.e., strong family history, neither a full-term delivery nor a first delivery after age 30). Randomization into the control or dietary study group will occur after appropriate baseline determinations are made. Clinical and dietary follow-up will continue for a minimum of five years. Patient accrual and compliance with the study diet are viewed as the major concerns. Even with strict compliance to the study, no risk or adverse affects are anticipated.